CO₂ Adsorption in Fe₂(dobdc): A Classical Force Field Parameterized from Quantum Mechanical Calculations

Carbon dioxide adsorption isotherms have been computed for the metal–organic framework (MOF) Fe₂(dobdc), where dobdc^4– = 2,5-dioxido-1,4-benzenedicarboxylate. A force field derived from quantum mechanical calculations has been used to model adsorption isotherms within a MOF. Restricted open-shell Møller–Plesset second-order perturbation theory (ROMP2) calculations have been performed to obtain interaction energy curves between a CO₂ molecule and a cluster model of Fe₂(dobdc). The force field parameters have been optimized to best reproduced these curves and used in Monte Carlo simulations to obtain CO₂ adsorption isotherms. The experimental loading of CO₂ adsorbed within Fe₂(dobdc) was reproduced quite accurately. This parametrization scheme could easily be utilized to predict isotherms of various guests inside this and other similar MOFs not yet synthesized

The Mechanism of Carbon Dioxide Adsorption in an Alkylamine-Functionalized Metal–Organic Framework

The mechanism of CO₂ adsorption in the amine-functionalized metal–organic framework mmen-Mg₂(dobpdc) (dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate; mmen = <i>N</i>,<i>N</i>′-dimethylethylenediamine) was characterized by quantum-chemical calculations. The material was calculated to demonstrate 2:2 amine:CO₂ stoichiometry with a higher capacity and weaker CO₂ binding energy than for the 2:1 stoichiometry observed in most amine-functionalized adsorbents. We explain this behavior in the form of a hydrogen-bonded complex involving two carbamic acid moieties resulting from the adsorption of CO₂ onto the secondary amines

Reversible CO Binding Enables Tunable CO/H₂ and CO/N₂ Separations in Metal–Organic Frameworks with Exposed Divalent Metal Cations

Six metal–organic frameworks of the M₂(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobdc^4– = 2,5-dioxido-1,4-benzenedicarboxylate) structure type are demonstrated to bind carbon monoxide reversibly and at high capacity. Infrared spectra indicate that, upon coordination of CO to the divalent metal cations lining the pores within these frameworks, the C–O stretching frequency is blue-shifted, consistent with nonclassical metal-CO interactions. Structure determinations reveal M–CO distances ranging from 2.09(2) Å for M = Ni to 2.49(1) Å for M = Zn and M–C–O angles ranging from 161.2(7)° for M = Mg to 176.9(6)° for M = Fe. Electronic structure calculations employing density functional theory (DFT) resulted in good agreement with the trends apparent in the infrared spectra and crystal structures. These results represent the first crystallographically characterized magnesium and zinc carbonyl compounds and the first high-spin manganese­(II), iron­(II), cobalt­(II), and nickel­(II) carbonyl species. Adsorption isotherms indicate reversible adsorption, with capacities for the Fe, Co, and Ni frameworks approaching one CO per metal cation site at 1 bar, corresponding to loadings as high as 6.0 mmol/g and 157 cm³/cm³. The six frameworks display (negative) isosteric heats of CO adsorption ranging from 52.7 to 27.2 kJ/mol along the series Ni > Co > Fe > Mg > Mn > Zn, following the Irving–Williams stability order. The reversible CO binding suggests that these frameworks may be of utility for the separation of CO from various industrial gas mixtures, including CO/H₂ and CO/N₂. Selectivities determined from gas adsorption isotherm data using ideal adsorbed solution theory (IAST) over a range of gas compositions at 1 bar and 298 K indicate that all six M₂(dobdc) frameworks could potentially be used as solid adsorbents to replace current cryogenic distillation technologies, with the choice of M dictating adsorbent regeneration energy and the level of purity of the resulting gases

Design of a Metal–Organic Framework with Enhanced Back Bonding for Separation of N₂ and CH₄

Gas separations with porous materials are economically important and provide a unique challenge to fundamental materials design, as adsorbent properties can be altered to achieve selective gas adsorption. Metal–organic frameworks represent a rapidly expanding new class of porous adsorbents with a large range of possibilities for designing materials with desired functionalities. Given the large number of possible framework structures, quantum mechanical computations can provide useful guidance in prioritizing the synthesis of the most useful materials for a given application. Here, we show that such calculations can predict a new metal–organic framework of potential utility for separation of dinitrogen from methane, a particularly challenging separation of critical value for utilizing natural gas. An open V­(II) site incorporated into a metal–organic framework can provide a material with a considerably higher enthalpy of adsorption for dinitrogen than for methane, based on strong selective back bonding with the former but not the latter